The present invention relates to fluid-coupling devices of the type that include both a fluid operating chamber and a fluid reservoir chamber. The fluid-coupling devices often have valving, which controls the quantity of fluid entering and exiting the operating chambers thereof. The devices may be electronically or mechanically controlled.
Although the present invention may be used advantageously in fluid-coupling devices having various configurations and applications, it is especially advantageous in a coupling device of the type used to drive a radiator cooling fan of an internal combustion engine, and will be described in connection therewith.
Fluid-coupling devices (“fan drives”) of the viscous shear type have been popular for many years for driving engine cooling fans, primarily because their use results in substantial saving of engine horsepower. The fluid-coupling devices typically operate in an engaged, relatively higher speed condition, only when cooling is needed. The fluid-coupling devices operate in a disengaged, relatively lower speed condition, when little or no cooling is required. Today, electrically actuated viscous fan drives are commonplace because they can be precisely controlled between engaged, partially engaged, and disengaged modes to control output at a given fan speed as determined by the vehicle's engine computer.
Controllable viscous fan drives, which are electrical or mechanical based, modulate fan drive output speed by controlling the balance of “fill” and “scavenge”. The term “fill” refers to the amount and rate of viscous fluid entering the working chamber from the fluid reservoir. The term “scavenge” refers to the amount and rate of fluid reentering the fluid reservoir from the working chamber. To control this balance, a rotary or axial type valve arm is utilized to vary the restriction of viscous fluid flow entering the working chamber by uncovering or covering one or more fill ports in the fluid reservoir. This control is often based upon engine operating parameters.
Some of the primary factors that control the fan drive at a steady state output speed are the working chamber geometry, the fluid viscosity, and the fluid volume or amount of fluid in the working chamber. The working chamber geometry is fixed by design and the fluid viscosity is fixed at steady state speeds. Thus, the output speed is dependent on the fluid volume. To increase output speed, the fill rate is increased to exceed the scavenge rate and to decrease output speed the opposite occurs.
Since the engagement of the fan drive is dependent upon the amount of viscous fluid entering the operating chamber through the fill port(s) at a given input speed and valve arm position, modulation robustness of the fan drive is inherently limited because the fan drive lacks active control over the rate in which viscous fluid exits the fluid operating chamber. Consequently, fan drive engagement, disengagement, and pumpout performance, especially at low input speeds, is adversely affected.
Thus, there exists a need for an improved viscous fan drive system and associated control technique that overcomes the drawbacks and limitations associated with prior systems.